It is known that a shock absorber for an airplane landing gear leg can be implemented in the form of a cylinder and a rod that slides inside said cylinder, said rod including a bottom that delimits a lower hydraulic fluid chamber which communicates via a diaphragm with an upper hydraulic fluid chamber adjacent to a chamber containing gas under pressure and formed in the top portion of the cylinder.
In certain situations, when the airplane is stationary or moving slowly over the ground, it is desirable to be able to modify the attitude of the airplane, i.e. the inclination of its longitudinal axis.
One possible approach then consists in attempting to change the length of its nose landing gear without altering its main landing gear: if the nose landing gear can be lengthened, then the attitude desired for an airplane which is stationary or which is moving slowly on the ground can be achieved.
Under such circumstances, it is advantageous to have nose landing gear capable of being lengthened.
Nevertheless, it is necessary to avoid confusing lengthening the landing gear while the airplane is stationary or moving slowly on the ground, and lengthening the landing gear so as to enable it to run over rough ground and even to pass over obstacles that are large in size. In the latter circumstance, the idea is to change the isothermal response curve of the shock absorber (variation in shock absorber force as a function of retraction stroke), e.g. by providing a structure such that the shock absorber has a single chamber on landing, and two chambers when taxiing (after the landing gear has been lengthened), as described in the document FR-A-2 601 097.
The shock absorber described in that document thus includes a moving disk that delimits the top of a high-pressure gas chamber whose bottom is delimited by a piston whose rod passes through the moving disk, and a bottom hydraulic fluid chamber which is delimited by said piston and by the end of the sliding rod, and which is fed from a controllable source connected to the hydraulic power generator of the airplane. The structure of that shock absorber provides the desired objective, i.e. it passes freely over bumps, and as a result, such a shock absorber is unsuitable for the sole purpose of statically lengthening the landing gear when the airplane is stationary or moving slowly on the ground.
Static lengthening of the landing gear for an airplane that is stationary or that is moving slowly on the ground, consists merely in generating a force equivalent to the static load on the landing gear so as to raise the cylinder of the shock absorber relative to its sliding rod (which rod is stationary, such that lengthening the shock absorber by moving out its sliding rod causes the cylinder or cylinder portion of said shock absorber to be raised).